Wireless communications systems are frequently implemented as one or more communications cells. Each cell normally includes a base station which supports communications with mobile nodes that are located in, or enter, the communications range of the cell's base station. Within a cell or a sector of a cell, the unit of communications resource is a symbol, e.g., QPSK or QAM transmitted on one frequency tone for one time slot in an orthogonal frequency division multiplexed (OFDM) system. The total available communication resource is divided into a number of such symbols (units) which can be used for communicating control and data information between a base station and one or more mobile nodes in the cell and tends to be limited. Control signals transmitted between a base station and a mobile node may be transmitted in two possible directions, i.e., from the base station to the mobile node or from the mobile node to the base station. Transmission of signals from the base station to the mobile is often called a downlink. In contrast, transmission from the mobile to the base station is commonly referred to as an uplink.
In order to provide efficient use of limited communications resources, base stations may allocate different numbers of tones to different mobile nodes depending on the devices' bandwidth needs. In a multiple access system, several nodes may be transmitting data, e.g., in the form of symbols, to a base station at the same time using different tones. This is common in OFDM systems. In such systems, it is important that symbols from different mobile nodes arrive at the base station in a synchronized manner, e.g., so the base station can properly determine the symbol period to which a received symbol belongs and signals from different mobile nodes do not interfere with each other. As mobile nodes move in a cell, transmission delay will vary as a function of the distance between a mobile node and a base station. In order to make sure that transmitted symbols will arrive at a base station from different mobile nodes in synchronized manner, timing control signals, e.g., feedback signals, may be and in many cases are, transmitted to each active mobile node of a cellular system. The timing control signals are often specific to each device and represent, e.g., timing corrections of offsets to be used by the device to determine symbol transmission timing. Timing control signaling operations include, e.g., monitoring for timing control signals, decoding received timing control signals, and performing timing control update operations in response to the decoded received timing control signals.
Timing control signals can be particularly important in systems where there are a large number of mobile nodes. In order to avoid interference from a mobile node due to timing miss synchronization, it may be necessary to establish timing synchronization and control before allowing a mobile node to transmit data, e.g., voice data, IP packets including data, etc. to a base station.
In addition to managing limited resources such as bandwidth, power management is often a concern in wireless communications systems. Mobile nodes, e.g., wireless terminals, are often powered by batteries. Since battery power is limited, it is desirable to reduce power requirements and thereby increase the amount of time a mobile node can operate without a battery recharge or battery replacement. In order to minimize power consumption, it is desirable to limit the amount of power used to transmit signals to a base station to the minimal amount of power required. Another advantage of minimizing mobile node transmission power is that it has the additional benefit of limiting the amount of interference that the transmissions will cause in neighboring cells which will often use the same frequencies as an adjoining cell.
In order to facilitate transmission power control, power control signaling, e.g., a feedback loop, may be established between a base station and a mobile node. Power control signaling often takes place at a much faster rate than the timing control signaling. This is because power control signaling attempts to track variations in the signal strength between the base station and the mobile nodes and this can typically vary on the scale of milliseconds. The timing control needs to take into consideration changes in the distance between base station and the mobile nodes and this tends to vary on a much slower scale, typically hundreds of milliseconds to seconds. Thus the amount of control signaling overhead for power control tends to be much more than that for timing control.
In addition to timing and power control signaling, other types of signaling may be employed. For example mobile nodes in addition may also signal on an uplink the quality of the downlink channel. This may be used by the base station to determine the communication resource allocation to allow for the transfer of data packets from the base station to the mobile. Such downlink channel quality reports allows a base station to determine which mobile node to transmit to and if a mobile node is chosen then the amount of forward error correction protection to apply to the data. These downlink channel quality reports generally are signaled on a similar time scale as the power control signaling. As another example, signaling may be used to periodically announce a mobile node's presence in a cell to a base station. It can also be used to request allocation of uplink resources, e.g., to transmit user data in a communications session. Shared as opposed to dedicated resources may be used for such announcements and/or resource requests.
Signaling resources, e.g., time slots or tones, may be shared or dedicated. In the case of shared time slots or tones, multiple devices may attempt to use the resource, e.g., segment or time slot, to communicate information at the same time. In the case of a shared resource, each node in the system normally tries to use the resource on an as needed basis. This sometimes results in collisions. In the case of dedicated resources, e.g., with time slots and/or tones being allocated to particular communications device or group of devices to the exclusion of other devices for a certain period of time, the problem of possible collisions is avoided or reduced. The dedicated resources may be part of a common resource, e.g., a common channel, where segments of the channel are dedicated, e.g., allocated, to individual devices or groups of devices where the groups include fewer than the total number of mobile nodes in a cell. For example, in the case of an uplink time segments may be dedicated to individual mobile nodes to prevent the possibility of collisions. In the case of a downlink, time slots may be dedicated to individual devices or, in the case of multicast messages or control signals, to a group of devices which are to receive the same messages and/or control signals. While segments of a common channel may be dedicated to individual nodes at different times, over time multiple nodes will use different segments of the channel thereby making the overall channel common to multiple nodes.
A logical control channel dedicated to an individual mobile node may be comprised of segments of a common channel dedicated for use by the individual mobile node.
Dedicated resources that go unused may be wasteful. However, shared uplink resources which may be accessed by multiple users simultaneously may suffer from a large number of collisions leading to wasted bandwidth and resulting in an unpredictable amount of time required to communicate.
While timing and power control signals and downlink channel quality reports are useful in managing communications in a wireless communications system, due to limited resources it may not be possible for a base station to support a large number of nodes when power control, and other types of signaling need to be supported on a continuous basis for each node in the system.
In view of the above discussion, it is apparent that there is a need for improved methods of allocating limited resources to mobile nodes to permit a relatively large number of nodes to be supported by a single base station with limited communications resources. It is desirable that at least some methods of communications resource allocation and mobile node management take into consideration the need for timing control signaling and the desirability of power control signaling in mobile communications systems.